Controlled formation of optically reflective and electrically conductive silvered surfaces on polyimide film via a direct ion-exchange self-metallization technique using silver ammonia complex cation as the precursor

Article abstract of DOI:10.1021/jp711373p

Double-surface-silvered polyimide films have been successfully fabricated using silver ammonia complex cation ([Ag(NH₃)₂] ⁺) as the silver precursor and 3,3a?2,4,4a?2- benzophenonetetracarboxylic dianhydride/4,4a?2-oxidianile- (BTDA/ODA-) based poly(amic acid) (PAA) as the polyimide precursor via a direct ion-exchange self-metallization technique. The process has been clarified to involve the loading of silver(I) into PAA via ion exchange, the thermally induced reduction of silver(I) to silver(O) and the concomitant imidization of PAA to polyimide upon thermal treatment, the subsequent silver-catalyzed and oxygen-assisted decomposition of the polyimide overlayer, and the self-accelerated aggregation of silver clusters on the film surface to produce well-defined surface silver layers. By employing [Ag(NH₃)₂]⁺ solution with a concentration of only 0.01 M and an ion-exchange time of no more than 10 min, the controlled formation of highly reflective and conductive silver surfaces upon thermal treatment at 300 ?°C for less than 4.5 h indicates that the present work provides an efficient route and an effacious silver species for polyimide surface metallization. Although the alkaline characteristics of [Ag(NH₃)₂]⁺ have a strong hydrolysis effect on the polyimide precursor chains, the final metallized films retain the key mechanical and thermal properties of the pure polyimide. Films were characterized by ATR-FTTR, XPS, ICP-AES, SEM, TEM, DSC, TGA, reflectivity, conductivity, and mechanical measurements. ? 2008 American Chemical Society.

Full text of DOI:10.1021/jp711373p

J. Phys. Chem. B 2008, 112, 5575-5584
5575
Controlled Formation of Optically Reflective and Electrically Conductive Silvered Surfaces
on Polyimide Film via a Direct Ion-Exchange Self-Metallization Technique Using Silver
Ammonia Complex Cation as the Precursor
Shengli Qi, Zhanpeng Wu, Dezhen Wu,* and Riguang Jin
State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology,
Beijing 100029, China
ReceiVed: December 2, 2007; In Final Form: February 15, 2008
Double-surface-silvered polyimide films have been successfully fabricated using silver ammonia complex
cation ([Ag(NH3)2]+) as the silver precursor and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride/4,4′-
oxidianile- (BTDA/ODA-) based poly(amic acid) (PAA) as the polyimide precursor via a direct ion-exchange
self-metallization technique. The process has been clarified to involve the loading of silver(I) into PAA via
ion exchange, the thermally induced reduction of silver(I) to silver(0) and the concomitant imidization of
PAA to polyimide upon thermal treatment, the subsequent silver-catalyzed and oxygen-assisted decomposition
of the polyimide overlayer, and the self-accelerated aggregation of silver clusters on the film surface to produce
well-defined surface silver layers. By employing [Ag(NH3)2]+ solution with a concentration of only 0.01 M
and an ion-exchange time of no more than 10 min, the controlled formation of highly reflective and conductive
silver surfaces upon thermal treatment at 300 °C for less than 4.5 h indicates that the present work provides
an efficient route and an effacious silver species for polyimide surface metallization. Although the alkaline
characteristics of [Ag(NH3)2]+ have a strong hydrolysis effect on the polyimide precursor chains, the final
metallized films retain the key mechanical and thermal properties of the pure polyimide. Films were
characterized by ATR-FTIR, XPS, ICP-AES, SEM, TEM, DSC, TGA, reflectivity, conductivity, and mechanical
measurements.
1. Introduction
in situ method has been considered to be the most likely for
preparing surface-silvered polyimide films because it has the
advantages of processing simplicity and outstanding adhesion
at the polymer-metal interface. For typical representative
research works on this technique, see refs 10, 27, and 32-34.
Highly reflective and electrically conductive surface-silvered
polyimide films (maximum reflectivity, >97%; surface resistiv-
ity, <0.1 Ω/square) have been fabricated with this technique.
However, successes were realized only when complex silver
compoundswithbridgingâ-diketonateligandssuchas(1,1,1,5,5,5-
hexafluoroacetylacetonato) silver(I) (AgHFA) and (1,1,1-trif-
luoroacetylacetonato) silver(I) (AgTFA)swhich are usually very
unstable, need to be freshly prepared, and are quite expensives
were used as the silver precursor.13,33 Nevertheless, metallization
could be realized only on the air side of the film. Neither
reflectivity nor conductivity was observed on the glass side. In
addition, attempts to fabricate silvered polyimide films using
simple silver salts have never achieved real success. In
particular, films formed with silver nitrate are visually degraded
and brittle without mechanical usefulness.33,35
The characteristics of polyimide, including excellent me-
chanical strength, good thermal stability, high chemical resis-
tance, and easy patterning, make it an important engineering
material that could be used in many fields.1-6 Surface-silvered
polyimide (PI) films, which combine the outstanding chemical
and physical properties of the polyimide matrix and the excellent
reflective and conductive properties of the surface silver layers,
are widely attractive in the microelectronics and aerospace
industries.7-10 Potential applications include flexible printed
circuit boards (FPCBs),11,12 flexible conductive tape,13 magnetic
data storage,12,14,15 highly active catalysts,15-18 optics,16,19-22
microelectronic devices,16,23,24 electromagnetic interference
shielding filters,25 and highly reflective thin film reflectors and
concentrators in space environments for solar thermal propulsion
orlarge-scaleradiofrequencyantennasforspaceapplications.10,26-28
Several methods have been developed to produce surface-
metallized polymeric films. One of the most well-established
approaches is external deposition, which directly deposits a
metal phase onto a previously prepared substrate.10,22,29,30
However, formation of surface silver layers on polyimide films
with this method is unfeasible because silver is a passive metal
and the films are usually prepared with very poorly adhered
metallic layers.13,31
Because of the limitations of the above route, our group has
developed a novel direct-ion-exchange self-metallization method
aimed at fabricating surface-silvered polyimide films by em-
ploying simple silver compounds, especially silver nitrate, as
the precursor. The approach works by introducing metal ions
into pure poly(amic acid) (PAA), which contains many car-
boxylic acid groups (active components of an ion-exchange
reaction), through ion exchange of the damp-dry PAA films in
an aqueous silver(I) solution. Subsequent thermal treatment of
the silver(I)-doped PAA films under tension converts the
As an alternative, in the 1990s, Southward’s group reported
an in situ single-stage self-metallization protocol that can also
be conceptualized as inverse chemical vapor deposition.10 This
* To whom correspondence should be addressed. Tel.: +86-10-6442-
1693. Fax: +86-10-6442-1693. E-mail: wdz@mail.buct.edu.cn.
10.1021/jp711373p CCC: $40.75 © 2008 American Chemical Society
Published on Web 04/12/2008

5576 J. Phys. Chem. B, Vol. 112, No. 18, 2008
Qi et al.
SCHEME 1: Ideal Synthetic Protocol for Producing Metallized BTDA/ODA Polyimide Films Using Silver Ammonia
Complex Cation as the Precursor via the Direct Ion-Exchange Self-Metallization Process
precursor into its final polyimide form, with concomitant silver
reduction and aggregation giving metallic layers on the polymer
surfaces.36-38 This ion-exchange technique was demonstrated
to be a rather effective approach for the metallization of
polyimide films in our recent publications. In particular, double-
surface-silvered polyimide films with high reflectivity and
conductivity on both sides were fabricated. For the 3,3′,4,4′-
benzophenonetetracarboxylic acid dianhydride/4,4′-oxidianiline-
(BTDA/ODA-) based polyimide matrix, the silvered films were
prepared with optimum reflectivities of 71% and 84% and
surface resistances of 4.1 and 1.3 Ω/square on the upside and
underside, respectively, when silver nitrate (AgNO3) was
employed as the silver source.37 Similarly, by utilizing silver
fluoride (AgF), maximum reflectivities higher than 80% and
100% and electrical resistances of less than 0.6 and 0.2 Ω/square
on the upside and underside, respectively, were achieved.38
In the present study, we report our efforts on the formation
of silvered surfaces on BTDA/ODA-based polyimide films using
silver ammonia complex cations ([Ag(NH3)2]+) as the silver
precursor via the direct ion-exchange self-metallization tech-
nique. The ideal synthetic protocol is shown in Scheme 1. This
work was undertaken because the efficiencies of polyimide film
metallization in previous works obtained by utilizing AgNO3
and AgF as the silver precursor were not satisfactory. To
fabricate the above two films, BTDA/ODA poly(amic acid)
films were ion-exchanged in an aqueous 0.4 M AgNO3 solution
for 40 min and in an aqueous 0.1 M AgF solution (saturated
concentration, 0.14 M) for 20 min, respectively.37,38 Moreover,
the AgNO3-ion-exchanged film must be thermally treated at
300 °C for more than 9 h to achieve its optimum properties.37
However, the experimental results obtained here using [Ag-
(NH3)2]+ indicate that highly reflective and conductive silver
layers were successfully fabricated on the polyimide film using
more dilute silver(I) solution (0.01 M), a shorter ion-exchange
time (no more than 10 min), and a shorter thermal treatment
time (less than 4.5 h at 300 °C). Moreover, the final metallized
films were prepared with well-boned surface silver layers and
retained the key thermal and mechanical properties of the pure
polyimide film. The mechanisms associated with the efficient
formation of silver surfaces and the ion exchange between the
alkaline [Ag(NH3)2]+ and the PAA precursor films are discussed
in detail. The surface, structural, and morphological character-
istics of the resulting films were characterized by UV/vis
spectrometry, surface reflectivity measurements, attenuated total
reflection-Fourier transform infrared spectroscopy, X-ray dif-
fraction, scanning electron microscopy, transmission electron
microscopy, and X-ray photoelectron spectroscopy.
2. Experimental Section
2.1. Materials. 3,3′,4,4′-Benzophenonetetracarboxylic dian-
hydride (BTDA) was purchased from Acros Organics and used
without further purification. 4,4′-Oxidianiline (4,4′-ODA) was
obtained from Shanghai Research Institute of Synthetic Resins
and recrystallized from ethyl acetate prior to use. Dimethylac-
etamide (DMAc) (analytically pure, e0.1% water) was pur-
chased by Tianjin Fu Chen Chemicals Reagent Factory and used
after distillation. Ammonia solution (analytically pure, 25-28
wt %) was purchased from Tianjin No. 3 Chemical Reagent
Factory. Silver nitrate (AgNO3) (analytically pure, g99.8%
content) was produced by Beijing Chemical Works and used
as received. Silver ammonia complex cations ([Ag(NH3)2]+)
were easily prepared by adding dilute ammonia solution
dropwise to the aqueous AgNO3 solution until a transparent
solution was obtained. In the present work, a 0.01 M aqueous
[Ag(NH3)2]+ solution with pH values of 9.9-10.2 was used.
2.2. Preparation of Metallized Films. The BTDA/ODA
poly(amic acid) solution was prepared with a 1 mol % offset
of dianhydride at 20 wt % solids in DMAc. The inherent
viscosity of the synthesized resin was ca. 1.8 dL/g at 35 °C. By

Formation of Silvered Surfaces on Polyimide Film
J. Phys. Chem. B, Vol. 112, No. 18, 2008 5577
TABLE 1: XPS Surface Composition Data for the
Poly(amic acid) Films Ion-Exchanged in 0.01 M Aqueous
[Ag(NH3)2]+ Solution for Varying Timesa
spreading the homogeneous poly(amic acid) solution onto a
clean glass plate and allowing the solvent to evaporate, damp-
dry poly(amic acid) films with thicknesses of 40-45 µm and
DMAc contents in the range of 38.5-39.2 wt % were then
produced. These films were next peeled from the glass substrate
and treated in a 0.01 M aqueous [Ag(NH3)2]+ solution to load
silver ions into the matrix via ion exchange. For clarity, the
surface of the damp-dry PAA film in contact with the glass
substrate is referred to as the upside, and that exposed to the
atmosphere is referred to as the underside. After being rinsed
thoroughly with deionized water, the silver(I)-doped films were
thermally treated under tension in a forced-air oven. The thermal
cycles are heating over 1 h to 135 °C and holding for 1 h,
heating to 300 °C over 2 h, and holding the temperature constant
at 300 °C. Thermal curing cycloimidizes the poly(amic acid)
and simultaneously induces an internal silver reduction and then
aggregation to form silver metallic layers, as depicted in
Scheme 1.
2.3. Film Characterization. The amount of silver(I) loaded
into the PAA films was quantified with a Seiko Instruments
SPS 8000 inductively coupled plasma (ICP) atomic emission
spectrometer. The measurements were performed after the silver-
doped PAA films had been dissolved in a 5 wt % nitric acid
solution.
Attenuated total reflection-Fourier transform infrared (ATR-
FTIR) spectra of the films were collected using a Nicolet Nexus
670 IR spectrometer.
Reflectivity spectra (relative to a BaSO4 mirror set at 100%
reflectivity) were obtained on a Shimazu 2501PC UV/vis
spectrophotometer at an incidence angle of 8° in the 200-800
nm wavelength range. The values at 531 nm were selected to
represent the film’s reflectance in the visible light region.
Surface electrically resistivities were measured with a RTS-8
four-point probe meter produced by Guangzhou Semiconductor
Material Research Institute in China.
The pH values of the aqueous solution were measured with
a PHS-2C pH meter produced by Shanghai Kangyi Instrument
Co., Ltd. (SHKY). The pH meter was calibrated using both
aqueous phosphate mixture solution with a pH value of 6.86
and aqueous sodium tetraborate solution with a pH value of
9.18 prior to measurements.
Surface morphologies were recorded on a Hitachi S-4300
field-emission scanning electron microscope (FE-SEM) operat-
ing at 15 kV. The samples were coated with ca. 5 nm of
platinum prior to the measurements.
The cross-sectional morphology of the films was observed
using a Hitachi H-800 transmission electron microscope (TEM)
at an accelerating voltage at 200 kV. Samples for TEM
observations were prepared by adhering the film on a poly-
(vinyl chloride) plate and then cutting it perpendicular to the
film surface using an ultramicrotome. These thin sections
floating on a water bath were mounted onto the carbon-coated
TEM copper grids.
relative atomic concentration (%)
ion-exchange
time (min)
C 1s
O 1s
N 1s
Ag 3d
0 (pure PAA)
5
10
70.27
50.37
44.71
24.39
18.52
18.26
5.34
22.23
26.25
0.00
8.88
10.78
a Measurements were performed on the underside.
449C system at a heating rate pf 5 K min-1. Mechanical
properties were evaluated using an Instron-1185 system.
3. Results and Discussion
3.1. Ion Exchange. Poly(amic acid)s have significantly higher
cation-complexing properties than their imide forms, because
of the presence of many active carboxylic acid groups in the
macromolecules. This characteristic allows for in situ reactions
with the salts of noble metals having labile anions to generate
a metal-polymeric blends.16 In our recent publications,36-38 we
demonstrated that silver ions can be successfully introduced into
the poly(amic acid) films by ion exchange in aqueous silver(I)
solutions, and we suggested that the metal ions were loaded
through the coordination of the negatively charged polycar-
boxylate groups with the positive silver ions to form a silver
polycarboxylate salt, silver polyamate, in the precursor films.
However, as described earlier, the efficiencies of incorporating
metal ions using silver nitrate37 and silver fluoride38 are rather
disappointing, i.e., the silver(I) solution must have a high
concentration and the necessary ion-exchange time is too long.
Accordingly, an aqueous silver ammonia complex cation
solution was employed in the present work. The ideal chemistry
involved in the film preparation process, including ion exchange
and film metallization, is depicted in Scheme 1. This alkaline
silver cation was selected because we believe that its basic
characteristics should accelerate the dissociation of carboxylic
acid groups and consequently greatly promote their coupling
with silver ions.
However, because of the presence of amide groups in the
PAA macromolecules, the loading of silver via ion exchange
is inevitably accompanied by the hydrolysis and degradation
of the polymer chains, especially in alkaline environments. Thus,
in the present study, a [Ag(NH3)2]+ solution with a concentration
of only 0.01 M (pH 9.9-10.2) was selected, and the ion-
exchange time was confined to no more than 10 min. Neverthe-
less, as quantified by ICP atomic emission spectroscopy, the
silver(I) loaded into the PAA films reached 6.59 and 7.23 wt
% after only 5 and 10 min of treatment, which is significantly
higher than the 4.32 wt % obtained for films that were ion-
exchanged in a 0.4 M AgNO3 solution for 40 min.37 Cor-
respondingly, the XPS composition data in Table 1 also indicate
that the amount of silver ions on the PAA surfaces reached levels
of 8.88 and 10.78 atom %, respectively. These results demon-
strate that [Ag(NH3)2]+ is indeed a rather effective silver source
for ion exchange. Moreover, the rapid and highly efficient
loading of silver(I) into the precursor films can reasonably be
attributed to the alkaline nature of the [Ag(NH3)2]+ solution,
as mentioned previously.
X-ray diffraction (XRD) data were recorded on
a
D/Max2500VB2+/PC X-ray diffractometer (Rigaku, Tokyo,
Japan) in the 5-90° range at a scanning rate of 0.18°/s.
X-ray photoelectron spectroscopy (XPS) measurements were
performed on an ESCALAB 250 spectrometer (Thermo Electron
Corporation) equipped with a monochromatic Al KR X-ray
source. The spectra were collected at a takeoff angle of 45°.
The pressure in the analysis chamber was maintained at 2 ×
10-10 mbar or lower during each measurement.
The chemical state of the metal ions loaded into the precursor
films is always one of the primary concerns. Figure 1shows
the ATR-FTIR characterization results of the PAA films before
and after ion exchange. The appearance of a strong absorbance
peak at 1361 cm-1 and the broadening of the band near 1599
cm-1 in the IR spectra of the films immersed in a 0.01 M
Differential scanning calorimetry (DSC) and thermal gravi-
metric analysis (TGA) were conducted on a NETZSCH STA

5578 J. Phys. Chem. B, Vol. 112, No. 18, 2008
Qi et al.
Figure 1. ATR FT-IR spectra of the metal-free pure poly(amic aicd)
film and the silver(I)-doped precursor films prepared with ion-exchange
times of 5 and 10 min in 0.01 M [Ag(NH3)2]+ solution.
aqueous silver ammonia complex cation solution for 5 min
(Figure 1b) and 10 min (Figure 1c) indicate that metal
polycarboxylate salts were successfully formed in the precursor
films during the ion-exchange process.37,39-41 Initially, we
expected that this was the result of the successful coordination
of the silver ammonia complex cations with the carboxylic
groups and that the silver(I) was present in the form of [Ag-
(NH3)2]+ in the precursor films, as illustrated in Scheme 1. This
is not an unreasonable conclusion because [Ag(NH3)2]+ is a
quite stable silver(I) species. However, XPS results entirely
refuted this expectation. Figure 2 displays the XPS Ag 3d core-
level spectra for the PAA films ion-exchanged in (a) 0.4 M
AgNO3 solution for 40 min, (b) 0.1 M AgF solution for 20 min,
and (c) 0.01 M [Ag(NH3)2]+ solution for 5 min. As for the first
two films, the high-resolution spectrum for the alkaline-silver-
(I)-ion-exchanged film also gave Ag 3d signals at the same
binding energy position of about 368.8 eV. This suggests that
the chemical state of the loaded metal ions in the [Ag(NH3)2]+-
PAA film is the same as that in the AgNO3 and AgF films, i.e.,
the silver ions are mainly present in the form of silver
polycarboxylate (-COO-Ag+ pairs).
Figure 2. XPS Ag 3d core-level spectra for the PAA films ion-
exchanged in (a) 0.4 M AgNO3 solution for 40 min, (b) 0.1 M AgF
solution for 20 min, and (c) 0.01 M [Ag(NH3)2]+ solution for 5 min.
SCHEME 2: Possible Reaction Mechanism Suggested
for the Loading of Silver Ions into PAA through Ion
Exchange between Carboxylic Acid Groups and Silver
Ammonia Complex Cations
In light of these results, the ion-exchange reactions in Scheme
1 must be reconsidered, and possible silver-loading mechanisms
are proposed as in Scheme 2. It is suggested that the ammonia
molecules are released from the silver complex (reaction 1) and
subsequently interact with the carboxylic acid groups to generate
ammonium carboxylate pairs (-COO-NH4+) with better solu-
bility in water and better ability to dissociate (reaction 2).
Finally, the silver ions released from reaction 1 combine with
the ammonium carboxylate pairs to load the silver ions through
the formation of -COO-Ag+ pairs, as depicted in reaction 3.
This silver-loading mechanism seems to be reasonable because
with poly(amic acid) is risky because the alkaline nature of the
silver ion solution would result in hydrolysis of the precursor
films. The extensively increased nitrogen atomic concentrations
(Table 1) after ion exchange might be direct evidence of this.
However, it is fortunate that, under our present experimental
conditions, no seriously destructive effect on the precursor films
was observed. The remaining absorption peak at 1713 cm-1
attributed to the vibrational modes of the carboxylic acid and
the bands at 1662 and 1541 cm-1 corresponding to the
vibrational mode of the amide group, observed on the PAA films
after ion exchange (Figure 1b,c), reflect the amic acid charac-
teristics of the ion-exchanged film.42,43 The characteristic
functional groups present in basic PAA molecules retained their
integrity after being treated in the alkali silver solution.
the ion-exchange reaction between -COO-NH4 and Ag+ is
+
considered to be much faster than that between -COOH and
Ag+, which could logically account for the rapid and efficient
loading of silver(I) into PAA using silver ammonia complex
cation solution. In addition, the XPS data in Table 1 indicate
that the surface atomic concentration of nitrogen (i.e., the N 1s
signal) increased by a factor of 4-5 after ion exchange,
supporting the occurrence of the above-suggested reactions,
especially reaction 2.
3.2. Film Metallization. The ideal chemistry involved in the
film metallization process is depicted in Scheme 1. Thermal
curing converts the poly(amic acid) into its final polyimide form,
Although allowing silver ions to be loaded efficiently, the
utilization of [Ag(NH3)2]+ to perform ion-exchange reactions

Formation of Silvered Surfaces on Polyimide Film
J. Phys. Chem. B, Vol. 112, No. 18, 2008 5579
Figure 3. FE-SEM images of double-surface-silvered polyimide films
with an ion-exchange time of 10 min in 0.01 M aqueous [Ag(NH3)2]+
solution that were cured to 300 °C for (A) 2.5, (B) 3.5, and (C) 4.5 h.
(U, upside of the film; D, underside of the film.)
with concomitant silver reduction and aggregation giving the
silver-polyimide hybrid films. However, to obtain a flat and
compact film characterized by good mechanical properties, it
is necessary that the silver(I)-doped precursor be thermally
treated under uniform tension. Otherwise, curl, shrinkage, or
deformation would occur because of the thermoplastic charac-
teristics of the poly(amic acid) molecules.
In our initial work, efforts to achieve surface metallization
mainly focused on the films ion-exchanged in 0.01 M aqueous
[Ag(NH3)2]+ solution for 10 min. Figure 3A-C shows the
surface topographies of the produced silver-polyimide com-
posite films thermally treated to 300 °C for 2.5, 3.5, and 4.5 h,
respectively. As can be clearly observed, well-established silver
layers formed on both the upside and underside surfaces of the
hybrid films, particularly after 3.5 h at 300 °C (Figure 3B,C).
This suggests that surface silver metallization on the polyimide
matrix was successfully achieved using silver ammonia complex
cation as the silver source via the direct ion-exchange self-
metallization process.
Figure 4. Development of reflectivity on the upside and underside
surfaces as a function of the cure protocol for the hybrid films ion-
exchanged for (A) 10 and (B) 5 min in a 0.01 M aqueous [Ag(NH3)2]+
solution. (Time zero is curing at 135 °C for 1 h.)
TABLE 2: Surface Resistivity Data (Ω/square) for the 10-
and 5-min-Ion-Exchanged BTDA/ODA Films Cured to
300 °C for Varying Times
10-min-ion-exchanged
5 min-ion-exchanged
thermal history
upside
underside
upside
underside
300 °C for 1 h
300 °C for 2 h
300 °C for 2.5 h
300 °C for 3.5 h
300 °C for 4.5 h
300 °C for 5.5 h
>106
3.2
2.0
0.8
0.6
>106
7.0
4.5
0.7
0.8
>106
6.0
1.7
0.7
0.6
>106
5.5
0.7
0.4
0.6
0.8
0.6
0.4
0.4
Reflectivity measurements were performed on both the upside
and underside surfaces of the metallized films cured at different
stages of the thermal cycle. Figure 4A shows plots of reflectivity
versus cure time/temperature for the 10-min ion-exchanged
hybrid films. It is exciting to note that maximum reflectivities
of 80.5% and 91.0% were achieved on the upside and underside,
respectively, of the metallized films that were thermally treated
for 4.5 h at 300 °C (Figure 3C). The variation of surface
electrical resistivity for the hybrid films with increasing cure
time/temperature is also reported in Table 2. For thermal
treatments of less than 300 °C for 1 h, the characterization data
indicate that conductivity was never achieved on either side of
the metallized films. However, films were soon fabricated with
surface resistance values of 3.2 and 7.0 Ω/square on the upside
and underside, respectively, after only an additional 1 h of
thermal treatment on the 10-min-ion-exchanged films (300 °C
for 2 h), implying the formation of very conductive surfaces.
As the thermal cure proceeds, the surface resistivity rapidly
decreased to less than 1 Ω/square, and for treatment at 300 °C
for 4.5 h, metallized films with surface electrical resistances of
0.6 and 0.8 Ω/square on the upside and underside, respectively,
were produced. Both the reflectance and resistance characteriza-
tion results indicate that the silver layers formed on the film
surfaces are highly reflective and conductive, the characteristics
that are the most important targets of our work.
Success was also achieved in our later work by shortening
the ion-exchange time in 0.01 M [Ag(NH3)2]+ solution to 5 min.
With maximum reflectivities of 81.8% and 93.1% and surface
resistances of 0.6 and 0.6 Ω/square on the upside and underside,
respectively, after being cured to 300 °C for 4.5 h, the 5 min-
ion-exchanged films also were well-metallized on both sides
with high reflectivity and conductivity, as indicated in Figure
4B and Table 2. Compared to the BTDA/ODA-AgNO3 silvered
films37 (optimum reflectivities of 71% and 84%, surface
resistances of 4.1 and 1.3 Ω/square) prepared with an ion-
exchange time of 40 min in 0.4 M aqueous AgNO3 solution
and cured to 300 °C for 9 h, the successful formation of silvered
surfaces with such desirable reflective and conductive properties
on both sides by employing a 0.01 M silver(I) solution with an
ion-exchange time of merely 10 or 5 min and a thermal
treatment time of only 4.5 h at 300 °C strongly suggests that
silver ammonia complex cation is a rather effective silver species
for polyimide film metallization.

5580 J. Phys. Chem. B, Vol. 112, No. 18, 2008
Qi et al.
Figure 5. FE-SEM images of the upside and underside surfaces of (A) the pure polyimide film and (B-D) the 10-min-ion-exchanged hybrid films
cured to (B) 135 °C for 1 h, (C) 300 °C for 0 h, and (D) 300 °C for 1 h. (U, upside of the film; D, underside of the film.)
Nevertheless, the plots in Figure 4 indicate that the surface
properties of the metallized films develop gradually with
increasing cure time and temperature in a stepwise manner. For
thermal treatments of less than 300 °C for 1 h, both the upside
and underside of the hybrid films exhibited rather disappointing
reflectivities in the range of 3-8% (Figure 4A), which are not
much different from the ca. 6% reflective surfaces of pure
polyimide films. A rapid increase in reflectivity began for a
thermal treatment of 300 °C for 2 h, and in no more than 2 h,
highly reflective surfaces were achieved. For the same time
treatment (300 °C for 2 h), the films prepared were also
conductive, as shown in Table 2, and the surface resistances
rapidly decreased to less than 1 Ω/square with increasing cure
time. Further thermal treatment at 300 °C did not improve the
reflectivity and conductivity significantly, but seemed to be
necessary to achieve the optimum surface properties.
SEM micrographs, reflecting the variation of the surface
morphologies of the 10-min-ion-exchanged hybrid films sub-
jected to thermal treatments of less than 300 °C for 1 h, are
shown in Figure 5. As a reference, SEM analysis was also
performed on the pure polyimide films (Figure 5A). After being
heated at 135 °C for 1 h, the films exhibited almost the same
completely transparent light-yellow appearance as the pure
BTDA/ODA precursor films. The SEM images in Figure 5B
show two rather plain surfaces without the detection of any silver
particles. Also, the X-ray diffraction pattern in Figure 6 for the
films subjected to a thermal treatment of 135 °C for 1 h exhibits
no reflections expected for the reduced silver metal, consistent
with the low reflectivities observed for this time and temperature.
In contrast, the XRD patterns soon gave very distinct peaks
characteristic of the crystalline state of face-centered-cubic (fcc)
silver metal after the temperature was raised to 218 °C.
Simultaneously, the silver(I)-doped films turned dark blue/black
and became completely nontransparent, which would most likely
correspond to silver(I) reduction and subsequent aggregation
in the polymer matrix. Further thermal treatment converts the
films to blue at 300 °C and next to green after thermal treatment
at 300 °C for 1 h.
Figure 6. XRD patterns of the silvered polyimide films after thermal
treatments of different times/temperatures.
XRD peak in the silver (111) region did not narrow significantly
with increasing cure time and temperature, suggesting a rather
slow increase in the dimension of the silver particles upon
thermal treatments of less than 300 °C for 1 h. The SEM images
in Figure 5C,D also demonstrate the reduction of silver(I) and
aggregation of silver atoms at the film surfaces. Many small
spherical silver particles with diameters on the order of ∼20
nm or less were uniformly distributed on the film surfaces,
especially for the films cured to 300 °C for 1 h (Figure 5D).
Yet, these small aggregates were all island-like and separated
from each other, consistent with the lack of conductivity at this
point. However, with reflectivities of only 5.7% and 7.7% on
the upside and underside, respectively (Figure 4A), the films
remained at a very low reflective level.
XPS measurements (with a probing depth of about 5-7.5
nm) were performed on the metallized films. As reported in
Table 3, the film subjected to a thermal treatment of 300 °C
for 1 h displays a surface silver atomic concentration of only
3.94%, whereas its carbon atomic content is approximately 66%,
close to that of the pure polyimide, indicating that the surface
The frequent color changes at this thermal stage are suggested
to originate from the continuous aggregation of the reduced
silver atoms to form larger silver particles. However, the main

Formation of Silvered Surfaces on Polyimide Film
J. Phys. Chem. B, Vol. 112, No. 18, 2008 5581
TABLE 3: XPS Surface Composition Data for the
10-min-Ion-Exchanged Hybrid Films Cured at Different
Thermal Stagesa
relative atomic concentration (%)
film sample
C 1s
O 1s
N 1s
Ag 3d
pristine PI, 300 °C for 5 h
PI-Ag, 300 °C for 1 h
PI-Ag, 300 °C for 2.5 h
PI-Ag, 300 °C for 4.5 h
74.32
65.99
57.67
35.52
21.59
17.91
17.23
24.19
4.09
12.16
15.87
16.10
0.00
3.94
9.23
24.18
a Measurements were performed on the underside.
TABLE 4: Thermal and Mechanical Properties of the
Silvered Polyimide Films
10%-weight-loss
ion-
tensile
temperature (°C)
exchange strength modulus percent
film sample
time (min) (MPa) (MPa) elongation in N2
in air
571
pure PI at 300 °C
for 5 h
PI-Ag at 300 °C
for 4.5 h
PI-Ag at 300 °C
for 4.5 h
0
5
137.6 2932.5
110.7 3018.2
102.8 3193.6
14.7
8.6
565
598
596
411
393
10
6.3
of the metallized films is dominated by the organic species.
Thus, the observation of very poor reflectivities on hybrid film
surfaces with so many attached silver particles (Figure 5D) is
reasonable because the coverage of a thin polymer overlayer
that strongly absorbs in the visible light regions on the top film
surfaces would greatly compromise the reflective features of
the silver particles aggregated on the surfaces.
However, one initially confusing aspect is that the surface
atomic concentrations indicated by the silver 3d peaks, and also
the N 1s peaks, of the hybrid films substantially decreased after
thermal treatment, relative to those of the silver(I)-PAA
precursor films. As shown in Table 1, after ion exchange in a
0.01 M aqueous [Ag(NH3)2]+ solution for 10 min, the PAA
film had a surface elemental composition of 10.78 atom % Ag,
26.25 atom % N, and 44.71 atom % C. Compared to the pure
PAA films (Table 1), the decreased C concentration and the
increased N content on the silver(I)-PAA surface can be
ascribed to the occurrence of PAA hydrolysis during ion
exchange and the reaction of carboxylic acid groups with
ammonia molecules introducing a large number of carboxylic
ammonium (-COO-NH4+) pairs into the macromolecule
chains. In contrast, upon thermal treatment, the Ag and N atomic
concentrations decreased to 3.94 and 12.16 atom %, respectively,
with a concomitant increase in the C atomic content to
approximately 66 atom %, as listed in Table 4 for the film
subjected to a thermal treatment of 300 °C for 1 h. It is probable
that the process of thermal cycloimidization of the silver(I)-
Figure 7. (A) DSC and (B) TG analyses of the silvered polyimide
films cured at 300 °C for 4.5 h.
concentration from 3.94 to 9.23 atom %, accompanied by a
decrease in the carbon content from 65.99 to 57.67 atom %,
indicating a discernible decomposition of the surface polymer
overlayer. A rapid increase in reflectivity was observed near
this point, and simultaneously, surface conductivity was ob-
tained, as reported in Figure 4 and in Table 2. SEM micrographs
in Figure 3A for the film cured to 300 °C for 2.5 h display
completely different topographies from that of the prematurely
cured films in Figure 5. Many larger silver particles were
exposed and blended with the polymer frameworks on the film
surfaces, which might account for the increased reflectivity and
the conductivity. However, the fabrication of metallized films
with such good electrical conductivities at such an early stage
(300 °C for 2 or 2.5 h) is amazing, especially when the
reflectivity was still at such a low level, because the appearance
of electrical conductivity usually occurs after the achievement
of high reflectivity and after a long thermal treatment time, as
reported in the literature.13,33,37 Southward and Thompson32
suggested that the formation of conductive surfaces arises from
the degradation of the intervening polymers between the metal
particles to allow them to come into contact with each other.
In the present system, although the ion-exchanged PAA films
were successfully converted into the imide form after curing
(as demonstrated by IR spectroscopy, not shown), it is reason-
able that the polyimide structure might not be perfectly attained
because of the ammonia-catalyzed hydrolysis of PAA chains
occurring during ion exchange. Correspondingly, the formed
polyimide is relatively easier to degrade, resulting in conductive
films after shorter thermal treatment times.
doped PAA film to generate the polyimide matrix involves the
+
decomposition of the carboxylic ammonium (-COO-NH4
)
components, which would possibly eliminate the nitrogen
element by releasing free ammonia molecules from the system
and regenerating the carboxylic acid groups (-COOH). In
contrast, the concentration of Ag on the film surface following
thermal treatment probably decreases because the movements
of the polymer segments are more dynamic and much easier
than those of the as-aggregated silver particles near the surface
layer, especially at early stages of the thermal cycle where the
cure temperature is not very high, so that a thin polymer
overlayer is generated on the top surface. As a result, the C 1s
peak becomes predominant, and the atomic contents of Ag and
N are lowered.
Upon further thermal treatment of the film to 300 °C for 2.5
h, the XPS data in Table 3 show a sharp increase in the silver
A layer of faint whitish substance began to be appear on the
metallized film surfaces after treatment at 300 °C for 3.5 h,

5582 J. Phys. Chem. B, Vol. 112, No. 18, 2008
Qi et al.
Figure 8. Cross-sectional TEM images of 10-min-ion-exchanged hybrid films cured at 300 °C for (A) 1 and (B) 4.5 h. (U, upside; I, interior; D,
underside.)
and it became appreciable after an additional 1 h of thermal
treatment. This whitish substance was undoubtedly derived from
the degradation of surface polyimide layer induced by the
thermal catalytic effects of the silver agglomerations deposited
on the surface. XPS data shown in Table 3 for the film cured
at 300 °C for 4.5 h give a surface silver content as high as
24.18 atom % and a carbon concentration as low as 35.52 atom
%, confirming the occurrence of polymer decomposition.
Meanwhile, the optimum film, with reflectivities of 80.5% and
91.0% and surface resistances of 0.6 and 0.8 Ω/square for the
upside and underside, respectively, was obtained for the 10-
min-ion-exchanged system. The SEM images in Figure 3B,C
indicate that continuous and integrated silver layers formed on
both sides of the metallized films.
In accordance with the wide variations of the film surface
morphology in this final thermal stage, the XRD patterns in
Figure 6 show that the full width at half-maximum of the
strongest characteristic (111) reflection narrowed rather quickly
after treatment at 300 °C for 2 h and the five reflections for the
fcc silver crystallites became increasingly distinct, indicating
the occurrence of dramatic silver aggregation and the subsequent
formation of larger and more perfect silver crystallites. However,
it is strange that two unexpected peaks appeared in the XRD
patterns at about 9°and 12° for the films cured at 300 °C for 1
h or longer. Moreover, these peaks increased in intensity with
increasing cure time up to 2.5 h, after which they receded again.
It is presumed that these two unknown peaks can be attributed
to the restricted silver aggregation in the present silver-polymer
blend system producing imperfect, particularly porous silver
crystallites. However, this interpretation cannot be confirmed
with the available evidence, and further investigations are
needed.
exothermal peak appears in the range of 300-500 °C for the
metallized film measured under an air atmosphere (Figure 7A-1
and -2). The significant amount of heat released here is
suggested to arise from the aggregation of unstable silver
clusters, which had a high surface free energy and readily
combined together to form larger thermodynamically stable
aggregations. The stronger peak in Figure 7A-2 is due to the
higher silver concentration in the 10-min-ion-exchanged hybrid
film. However, in nitrogen environment, no pronounced thermal
effect was observed in the present experimental temperature
range (Figure 7A-3 and -4). These results indicate that silver
aggregation is an oxygen-assisted process.
Along with the liberation of heat, a significant mass loss
began to be observed in the TGA curves measured in air
atmosphere, as shown in Figure 7B-1 and -2. Thus, the function
of oxygen here should be that it helps to decompose the polymer
constituent at the surface, thereby encouraging the aggregation
of the small silver particles. However, once the silver aggrega-
tion is initiated, it becomes almost uncontrollable because the
released heat would accelerate further aggregation of the silver
particles following a chain-reacting mechanism. Enormous
variations in the film surface morphology then resulted, and
soon, well-defined silver layers with desirable surface perfor-
mances were formed, as observed in Figure 3. Cross-sectional
TEM also indicated the rapid formation of surface metal layers
upon silver aggregation, as shown in Figure 8. For thermal
treatment at 300 °C for 1 h (Figure 8A), the hybrid film was
metallized with only negligible silver layers on the film surfaces.
However, in no more than 4 h, near-perfect thin silver layers
formed on each film side, as shown in Figure 8B for the film
cured at 300 °C for 4.5 h. Therefore, we conclude that the
formation of highly reflective and conductive silver layers on
polyimide film is a result of the silver-catalyzed and oxygen-
DSC analysis was also performed on the hybrid films, and
the results are shown in Figure 7A. As can be observed, a strong

Formation of Silvered Surfaces on Polyimide Film
J. Phys. Chem. B, Vol. 112, No. 18, 2008 5583
Figure 9. FE-SEM micrographs recorded for the upside and underside surfaces of the silvered polyimide films (A) before and (B) after the
adhesion test. The film was initially ion-exchanged in 0.01 M [Ag(NH3)2]+ for 5 min and then cured to 300 °C for 4.5 h. (U, upside of the film;
D, underside of the film.)
Figure 10. Specular reflectance spectra as a function of light wavelength in the range of 200-800 nm recorded for the upside and underside
surfaces of the same film as in Figure 9 (A) before and (B) after the adhesion test.
assisted decomposition of the polymer overlayer and the self-
accelerated aggregation of silver clusters on the film surface.
TEM images in Figure 8B also suggest that our hybrid films
were fabricated with a sandwich structure, in which the silver
is mainly concentrated in the near-surface layer. This is very
favorable because the few metal particles remaining in the inner
polymer bulk with very small sizes (∼10 nm or less) make it
possible for the metallized films to retain the key mechanical
properties of the pristine polyimide, as shown later.
3.3. Adhesion Test. The adhesion between the metal layer
and the underlying polymer substrate is always a primary
concern when surface metallization is studied. As a qualitative
evaluation, mechanical debonding studies were performed on
the metallized films using commercial transparent adhesive tape.
After the film had been fixed on a substrate, the sample was
tested using 6-mm-wide adhesive tape by sticking it on the film
surface and then peeling it off. Macroscopic observation
indicated that no delamination occurred on either the upside or
the underside surface after debonding. For further validation,
SEM analysis was carried out on the metallized film before and
after the adhesion test, as shown in Figure 9. The fact that some
metal agglomerates were drawn away from the silvered surfaces,
as indicated by the peeling spots presented on the tested surfaces
(Figure 9B), cannot be ignored. However, the overall morphol-
ogies before and after the peeling test were not distinctly altered.
Continuous and net-like silver surfaces were still maintained.
Furthermore, the surface reflectance and resistance features
remained almost identical before and after adhesion test, as
shown in Figure 10. Thus, it can be concluded that the adhesion
at the silver-polyimide interface is quite acceptable for films
prepared using [Ag(NH3)2]+ via the direct ion-exchange self-
metallization process. Here, the good bonding strength is
attributed to the mechanical interlocking effect between silver
and polyimide, as reported in the literature.10,12,32
3.4. Mechanical and Thermal Characterization. As men-
tioned previously, the use of [Ag(NH3)2]+ is risky, because the
PAA chains might be seriously hydrolyzed by the alkaline silver-
(I) solution during ion exchange, thereby giving metallized films
with substantially degraded mechanical properties. This is true
but only to a limited extent in the present work. Table 4 displays

5584 J. Phys. Chem. B, Vol. 112, No. 18, 2008
Qi et al.
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As can be seen, the tensile strength of the polyimide film indeed
decreased (by about 25-35 MPa) after metallization, especially
for the 10-min-ion-exchanged films. However, the tensile
strength still remains at a level higher than 100 MPa, and the
modulus appears not to be diminished. Thus, it is confirmed
that the key mechanical properties of the pure polyimide are
retained, as previously anticipated from the TEM observations
in Figure 8B.
The thermal data listed in Table 4 suggest that the thermal
stability of the silver-doped films, as characterized by the
temperature at which 10% weight loss occurred, was never
lowered in a nitrogen environment. However, in air, the
degradation temperatures were considerably lower than those
of the host polyimide films. Such behavior is expected because
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4. Conclusions
Our present work demonstrates that silvered surfaces with
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BTDA/ODA-based polyimide matrix using [Ag(NH3)2]+ as the
silver precursor via the direct ion-exchange self-metallization
technique. With an ion-exchange time of no more than 10 min
in a 0.01 M [Ag(NH3)2]+ solution and with a thermal treatment
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Acknowledgment. The authors acknowledge financial sup-
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(NSFC, Project 50573007) and the program for New Century
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